Physics in free fall

Dropping supercold atoms may prove useful for understanding general relativity

In an experiment that puts the good old-fashioned egg drop to shame, European physicists dropped a small blob of ultracold atoms down a 146-meter-tall shaft. The result: no yolk on their face.

In the new study, researchers created a cloud of about 10,000 ultracold rubidium atoms, so still and chilly that the atoms fused into a quirky quantum object called a Bose-Einstein condensate. Then they dropped the stuff off a lofty needle-shaped tower in Bremen, Germany, that stands just 23 meters shorter than the Washington Monument.

Freely falling objects are essentially weightless. So the successful drop shows that researchers now have the ability to monitor quantum objects in near-zero gravity — which may lead to a deeper understanding of heavy topics such as general relativity, a consortium of German, English and French researchers report June 18 in Science.

The new study is “an impressive technological advance,” comments physicist Wolfgang Ketterle of MIT, who shared a Nobel Prize in 1995 for creating BECs in the lab. “I am thrilled to see in how many directions BEC research is taken — much further than I anticipated when BEC was discovered.”

One of the biggest challenges for the researchers was miniaturizing the jungle of complex equipment usually needed to create and maintain a BEC. Normally, making a BEC requires a big laboratory “crammed with mirrors and lenses and all sorts of optical components,” says physicist Paulo Nussenzveig of the University of São Paulo in Brazil, who coauthored an accompanying article in the same issue of Science. “These guys fit the equivalent equipment into a 60-by-60 centimeter by 2-meter capsule and sent it down 120 meters and it smashed at the bottom. It’s really, really amazing.”

A camera caught the BEC expanding before the capsule crashed into an 8-meter-deep pit of plastic balls. The atoms’ behavior in near-weightlessness largely agreed with theoretical predictions, although tiny stray magnetic perturbations caused the BEC to expand a little less than predicted, the team found.

Although the researchers haven’t discovered any new physics yet, they are hopeful that the system will soon be useful, says study coauthor Ernst Rasel of Leibniz Universität Hannover in Germany. “This is a modest first step towards something that you can do interesting experiments with,” he says.

He and his colleagues had Einstein’s famous elevator thought experiment in mind when they created the tower. A person inside a falling elevator would feel weightless, Einstein posited, since gravity’s tug is canceled by the downward acceleration. This insight, called the equivalence principle, led Einstein to develop the theory of general relativity. But just how general relativity applies to objects on the quantum scale remains a mystery.

Nussenzveig says that observing the behavior of the strange matter in microgravity may answer unresolved questions about how gravity works on very small, quantum scales. “To have a quantum theory of gravity, that is something that is still missing,” he says. Some researchers think that gravity might tug on different kinds of atoms in different ways, which a sensitive system like a free-falling BEC might be able to detect, he says.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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